1. Introduction to the Phylum Chordata
Honors Biology

2. Unifying Themes
1. Chordate evolution is a history of innovations that is built upon major invertebrate traits
They display many of the basic traits that first evolved in the invertebrates: bilateral symmetry, cephalization, segmentation, coelom, "gut" tube, etc.
2. Chordate evolution is marked by physical and behavioral specializations
For example the forelimb of mammals has a wide range of structural variation, specialized by natural selection
3. Evolutionary innovations and specializations led to adaptive radiations - the development of a variety of forms from a single ancestral group

3. Characteristics of the Chordates
Notochord, dorsal hollow nerve cord, pharyngeal gill slits, blocks of muscle, postanal tail

4. Characteristics of the Chordates
The notochord
All chordate embryos have a notochord, a stiff but flexible rod that provides internal support
Remains throughout the life history of most invertebrate chordates; among, present only in the embryos of vertebrate chordates

5. Dorsal Hollow Nerve Cord (=Spinal Cord)
Characteristics of the Chordates cont.
Dorsal Hollow Nerve Cord (=Spinal Cord)
A fluid-filled tube of nerve tissue that runs the length of the animal, dorsal to the notochord
Present in chordates throughout embryonic and adult life

6. Pairs of opening through the pharynx
Characteristics of the Chordates cont.
Pharyngeal gill slits
Pairs of opening through the pharynx
Invertebrate chordates use them to filter food
Juvenile fishes use them to them for breathing
In adult fishes the gill sits develop into true gills
In reptiles, birds, and mammals the gill slits are vestiges, occurring only in the embryo

7. Blocks of Muscle - Myotomes
Characteristics of the Chordates cont.
Blocks of Muscle - Myotomes
Surrounding the notochord and nerve cord are blocks of muscle - myotomes
Postanal Tail
The notochord, nerve cord, and the myotomes extend to the tail
Found at some time during a chordate's development

8. Invertebrate Chordates

9. SubPhylum Urochordata
Marine animals; some species are solitary, others are colonial.
Sessile as adults, but motile during the larval stages
Possess all 5 chordate characteristics as larvae
Settle head first on hard substrates and undergo a dramatic metamorphosis (e.g., tail, notochord, muscle segments, and nerve cord disappear)

10. SubPhylum Urochordata cont.
Adult body is covered by an outer envelope or tunic; composed of fibers of tunicin embedded in a mucopolysaccharide matrix
Tunic encloses a basket-like pharynx, that is perforated by gill slits
Tunicates are filter feeders; plankton is trapped in a sheet of mucus and cilia later direct the food-laden mucus to the stomach
Water leaves the animal via an excurrent siphon

11. Chordate Metamerism
Body segmentation (i.e. metamerism) appears to have evolved in two lineages of the chordates: the Cephalochordates and the Vertebrates; probably occurred after divergence from the Urochordates
However, segmentation in the chordates does not involve the coelom
The cephalochordates and the chordates movement is accomplished by contraction of muscle fibers that are arranged in segmented blocks - myotomes
Presumably, segmentation of muscles developed as an adaptation for undulatory swimming and rapid burrowing

12. SubPhylum Cephalochordata
Exclusively marine animals
Although they are capable of swimming, they usually are buried in the sand with only their anterior end being exposed

13. All chordate characteristics are present throughout their life history
SubPhylum Cephalochordata cont.
All chordate characteristics are present throughout their life history
They are filter feeders: inside of the oral hood is lined with cilia -wheel organ
These cilia, plus cilia in the pharynx help generate a water current
Water and suspended food particles pass through the oral hood, equipped with projections called cirri that strain larger particles
Feed by secreting a mucous net across the gill slits to filter out food particles that are present in the water.

14. Subphylum Vertebrata

15. General Characteristics
Exhibit all 5 chordate characteristics at sometime in their life history
Usually well cephalized, including a well developed brain and a number of anterior sensory structures
Brain is usually encased in a skull, made of hard bone or a cartilage.
In most vertebrates, the embryonic notochord is replaced by a vertebral column.
Possess a distinctive endoskeleton consisting of vertebral column, limb girdles, two pairs of jointed appendages, and a head skeleton
Muscles are attached to the skeleton to provide movement
Often have a muscular perforated pharynx
Closed circulatory system with a well developed muscular heart; blood is oxygenated as it flows through vascularized skin, gills or lungs.

16. Evolutionary Relationships of the Vertebrates
Earliest vertebrate fossils (jawless ostracoderm fishes; 500 mya) share many of the novel structures observed in the living vertebrates
Q. When and from where did these vertebrate characteristics evolve?
May have evolved from an invertebrate chordate lineage
This idea is supported by the discovery of a fossilized mid-Cambrain invertebrate chordate from the Burgess Shale formation - Pikaia
A ribbon shaped, somewhat fish-like creature about 5 cm in length
It possessed a notochord and the V-shaped myomeres
Resembles Amphioxus, and may very well be an early cephalochordate

17. Evolutionary Relationships of the Vertebrates cont.
Speculations regarding vertebrate ancestry have focused on living cephalochordates and tunicates
One hypothesis on the evolution of the vertebrates is Garstang's Hypothesis
It suggests that sessile tunicates were an ancestral stock that evolved a motile larval stage
Garstang speculated that at some point larvae failed to metamorphose into an adult, but developed gonads and reproduced in the larval stage
And with continued larval evolution a new group of free swimming animals evolved
Garstang called this process paedomorphosis, a term that describes the presence (or evolutionary retention) of juvenile or larval traits in the adult body

18. Vertebrate Classes Agnatha (Cyclostomata) Chondrichthyes Osteicthyes
Amphibia
Reptilia
Aves
Mammalia